Process of producing true gradation prints for making halftone printing plates



Dec. 1, 1925' 1,563,499

H. C. KNUDSEN PROCESS OF PRODUCING TRUE GRADATION PRINTS FOR MAKING HALFTONE PRINTING PLATES" Filed Aug. 27, 1919 TL Inven for Hugo C. Knudsen his Attorney;

Patented Dec. 1, 1925.

UNITED STATES PATENT-OF 1,5 3,499 FICE.

HUGO C. KNUDSEN, OF BROOKLYN, NEw YORK, ASSIGNOR TO.,AMZERICAN LITHO- GRAPHIC COMPANY, OF NEW -YORK, N. Y., A CORPORATION OF NEW YORK.

PROCESS OF PRODUCING TRUE GRADATION PRINTS FOR MAKING HALFTONE PRINTING PLATES.

Application filed August 27, 1919. Serial No. 320,140.,

To all whom it may concern:

Be it known that I, HUoo C. KNUDSEN, a citizenof the United States, and a resident of New York, borough of Brooklyn, in the county of Kings and State of New York, have invented certain new and useful Improvements in Processes of Producing True Gradation Prints for Makin Halftone Printing Plates, of which the ollowing is a specification.

This invention relates to processes'of producing half-tone printing plates, and has for its object to provide certain steps in a process of the character described which will result in half-tone plates having gradations of light and shade in true correspondence with the original.

In the drawingsaccompanying this specificationza Fig.1 is a diagrammatic view illustrating the arrangement of several elements employed by me in the practice of my improved process. Fig. 2 is an enlarged view of a portion of what is shown in Fig. 1'. Fig. 3 is a diagram to illustrate a phase of the process.

According to present practice if the halftone screen as 12, Fig. 1, is placed in contact with the negative plate 14. and the direct rays of an arc lamp are used the dots on the resultant positive will be practically of one size, but if the screen and plate are separated to permit the light to be difi'racted between them, the dots will vary in size. The nearer the plate and screen are to one another the less will be the variation in the size of the dots and the less the contrast in the resultant positive. As the distance between said plate and screen is increased the greater will be the contrast between the light and dark portions in the resultant positive, but on the other hand such increased separation results in an objectionable and proportionate loss of definition in the high lights. This is shown diagrammatically in Fig. 3, wherein the diagonal line 16, 17 represents the field of true gradation from black at 16 to white at 17. At best the present practice is able to obtain results only approximating the full tones of black and white. If the screen and plate are relatively. close to each other the resultant positive will approximate that mid-portion of the field of line 16, 17 out by the full line curve 18. If the screen and plate are further separated the resultant positive will approximate that mid-portion of the field of line 16, 17 cut by the dotted line curve 19. It will thus be seen that What is gained at one end is lost at the other, that is, what is gained inv contrast is -aid for in decreased definition in the high llghts, or the reverse.

By my improvements I am able to obtainresults showing true gradation from solid black to clear white.

To these ends I proceed as follows: The source of light should be an electric are light 9, or itsequivalent, and which should be as concentrated as possible. In front of said light is placed a pair of condenser lenses 10, of well-known character and preferably ground at'an angle of thirty degrees. After passing through the condenser lenses the rays of light are focused and then spread out by lens 11 so as to cover the area of the design portion of the continuous-tone design plate 14. This plate may be either positive or negative according to the character of the result desired. In the present description said design plate is. for convenience, assumed to be a negative. Said negative 14 is erected behind half-tone screen 12 and parallel thereto. both being normal to the general direc tion of the rays from light 9. The design film of negative 14 is on the rearward face of the plate, remote from screen 12. and against this film side of the negative is placed in close contact the light sensitized surface of a plate or paper 15. The sensitized coating on said plate or paper is preferably highly sensitive to light, so as to require a relatively short exposure to produce the desired result, and said coating is of a character to produce after developing, dense opaque portions Where exposed to the light.

Between screen 12 and negative 14 is preferably placed a glass plate 13 of proper thickness to separate said screen and the film face of the negative a predetermined distance. It will be understood that said glass 13 is interposed for convenience only and that said distance of se aration of screen 12 and negative 14 may e established and maintained by other convenient means.

This predetermined distance is ascertained as follows and is constant for all screens of the same cross ruling: Referrin to Fig. 2, which is. a greatly exaggerate diagrammatic cross-section of screen 12, negative 14 and sensitized-plate or paper 15, it will be understood that each of the openings, as 12",

in screen 12 acts as a minute lens and the .with said screen and the film face of negative 14 is located in this plane, so that the light rays passing through saidopening 12 will focus directly in the film face of the negative and in the sensitized face of plate 15, in close contact with said negative;

Y The negative 14 has true gradations. from black to white according to the original .from which it was made .and'that negative,

during the exposure of the sensitized paper 15 behind the screen and negative, determines the various densities of. the focal points of the light impinging on the li ht sensitizedv surface of the paper, so that w en this exposed paper 15 is developed after said exposure 15 com lete, it will carry dots of. various from nothing (white) to complete inte netrating size (black)- ac cording to the ensity of the negative.

The halftone print obtained in this manner possesses'absolutely true gradations corresponding to the'gradations of the negative and it therefore becomes possible by this process to obtain from color record and other negatives direct, without further photographic positives and screen negatives, photographic screened prints on paper, which may be used; for making printing plates onwhich no retouching or fine-etch mg is necessary. All corrections as to tone values necessary in the finished product may be done on the negative of-which the paper screened print is a true copy. Also I- have found that for ithe several screens noted herebelow the distances should closely approximate the following figures:

. For a line to the inch screen the distance should be 1 inch; for a 85 line to the mch screen the distance should be 4 inch, fora 100 lineto the inch screen the distance should be inch,,for a 120 line to the inch ,screen the distance should be A inch, for a;

133 line to the inch screen the distance should be 5inch, for a 150 line to theinch screen the distance should inch.

I claim: I Y I 1. That improvement in the artjof photography comprised in causing a beam of light to ass successively through a condenser, a ens, the'openings in a half-tone screen, and a design plate against a light sensitive surface in close contact with said design plate, whereby the design is photoscreen,

Ilumination over a given 1 graphically imprinted upon the light sensilight to pass successively through a condenser, a lens, the openings in a half-toneand a design plate, against a light sensitive surface in close contact with said design plate. a

3. That improvement in the art,of photography comprised in causing a beam oflight to pass successively through a condenser, a lens, the openings in a half tone screen, and a design plate, against a light sensitive surface in close contact with said design plate, the distance between said screen and design plate being equal to the focal length of the light rays which pass through the openings in said screen.

4. That improvement in the art of photography comprised in causing a beam of light to pass successively through a condenser, a lens, the openings in a half-tone screen, and a design plate against a light sensitive surface, said design plate being positioned relatively to said screen a distancevequal to the focal length of the light rays which pass through the openin in said screen, whereby the design is p otographically imprinted upon the light sensitive surface in true gradation corresponding with said design as modified by said screen.

5. That improvement in the art of photography comprised in causing a beam of light to pass successively through a'condenser, a lens, the openings in a half-tone,

screen, and a design plate against a light sensitive surface, said design'plate being positioned relatively to said screen a distance equal to the focal length of the light rays which pass through the openings .in said screen, whereby the design is photographicallyj imprinted upon the light sensitive surface in sharply focused dots of definite area. 6: That improvement in the art of mak-- ing half-tone prints the light from an electric arc lamp to pass through a condenser, whereby a uniform ilarea" is produced,

then causing said light to pass successively through a lens the openingsin a halftolie screenand 'a design. plate against a light sensitive surface in close contact with said design plate whereby the design is photographically imprinted upon the light-senscreen, resulting in a true gradation print comprised in causing corresponding with said design as modified 2 by said screen.

In witness whereof, I hereby afiix my signature this 25th day of August, 1919.

" HUGO C. KNUDSEN. 

